toolchain: introduce BR2_TOOLCHAIN_HAS_GCC_BUG_90620

toolchain: introduce BR2_TOOLCHAIN_HAS_GCC_BUG_90620

GCC fails building the haproxy package for the Microblaze architecture:

  http://autobuild.buildroot.org/results/64706f96db793777de9d3ec63b0a47d776cf33fd/

The gcc bug was originally reported gpsd:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90620

This gcc bug no longer appeared with gcc 8.x but reappeared in gcc
9.x, so we introduce a config symbol so that packages can work it
around by disabling optimization.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>